Noticing that people replying to other people I don't follow are showing up in my local timeline. So now I'm worrying: every time I have a conversation with someone, am I polluting everyone else's timeline with an unending series of replies?

That was one thing "the bird site" got right: you didn't see peoples' replies to people unless you followed both of them.

@csilverman Amarok gets this right. Replies are unlisted by default. I think it's just the web interface that defaults to public for replies.
@pete @csilverman The usual opinion on this here is that this is a good thing, as it makes it easier to jump in to other people's discussions and thus encourages conversation.
@lambadalambda @csilverman Thank you. I keep forgetting that mastodon and GNU social have been around for a looooooong time and of course all of this has been thought out and debated.
@pete @csilverman At least on GS there is a setting in the frontend where you can hide replies, but the default is the other behavior.